Triangular pyramid type cube-corner retroreflective element

ABSTRACT

To provide a retroreflective sheeting not only superior in high-brightness characteristic which is a basic optical characteristic generally requested for a triangular-pyramidal reflective element, that is, reflectivity represented by the reflectivity of the light incoming from the front of the triangular-pyramidal reflective element but also whose entrance angularity and rotation angularity are greatly improved.  
     A pair of triangular-pyramidal cube-corner retroreflective elements characterized in that the optical axis of the triangular-pyramidal reflective element pair tilts so that the angle formed between the optical axis and a vertical line extended from apexes (H1 and H2) of the elements to a bottom plane (S x -S x ′) ranges between 0.5 and 1.5° in the direction in which the difference (q−p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (S x -S x ′) up to base edges (x,x, . . . ) shared by the element pair and the distance from the intersection (P) of the vertical line and the bottom plane (S x -S x ′) up to the base edges (x,x, . . . ) shared by the element pair becomes plus (+) or minus (−).

TECHNICAL FIELD

[0001] The present invention relates to a triangular-pyramidalcube-corner retroreflective sheeting having a novel structure. Moreminutely, the present invention relates to a retroreflective elementsuch as a triangular-pyramidal cube-corner retroreflective element(hereafter merely referred to as a retroreflective element or areflective element) constituting a retroreflective body useful forreflectors such as signs including traffic signs and construction worksigns, license plates of vehicles such as automobiles and motorcycles,safety materials of clothing and life preservers, markings ofsignboards, and reflectors of visible-light, laser beams, orinfrared-ray reflective sensors, and an assembly of the retroreflectiveelements.

BACKGROUND ART

[0002] A retroreflective body for reflecting entrance light toward alight source has been well known so far and the reflective body usingits retroreflectivity is widely used in the above industrial fields.Particularly, a triangular-pyramidal cube-corner retroreflective body(hereafter also referred to as a CC reflective body) using theinternal-total-reflection theory such as a triangular-pyramidalcube-corner retroreflective element (hereafter also merely referred toas a triangular-pyramidal reflective element or CC reflective element)is remarkably superior to a retroreflective body using conventionalmicro glass beads in retroreflective efficiency of light and thereby,purposes of the triangular-pyramidal cube-corner retroreflective elementhave been increased year by year because of its superior retroreflectiveperformance.

[0003] However, though a conventional publicly-knowntriangular-pyramidal retroreflective element shows a preferableretroreflective efficiency when an angle formed between the optical axis(axis passing through the apex of a triangle equally separate from threefaces constituting a triangular-pyramidal cube-corner retroreflectiveelement and intersecting with each other at an angle of 90°) of theelement and an entrance ray is small because of the reflection theory ofthe element, the retroreflective efficiency is suddenly lowered (thatis, the entrance angle characteristic is deteriorated) as the entranceangle increases. Moreover, the light entering the face of thetriangular-pyramidal reflective element at an angle less than thecritical angle (α_(c)) meeting an internal-total-reflection conditiondecided in accordance with the refractive index of a transparent mediumconstituting the triangular-pyramidal reflective element and that of airreaches the back of the element without totally reflecting from theinterface of the element. Therefore, a retroreflective sheeting using atriangular-pyramidal reflective element generally has a disadvantagethat it is inferior in entrance angularity.

[0004] However, because a triangular-pyramidal retroreflective elementcan reflect light in the direction in which the light enters over thealmost entire surface of the element, reflected light does not reflectby diverging at a wide angle due to spherical aberration like the caseof a micro-glass-bead reflective element. However, the narrow divergentangle of the reflected light easily causes a trouble that when the lightemitted from a head lamp of an automobile retroreflects from a trafficsign, it does not easily reach, for example, eyes of a driver present ata position separate from the optical axis of the head lamp. The abovetype of the trouble increases more and more (that is, observationangularity is deteriorated) because an angle (observation angle) formedbetween the entrance axis of rays and the axis connecting a driver witha reflection point increases.

[0005] Many proposals have been made so far for the above cube-cornerretroreflective sheeting, particularly for a triangular-pyramidalcube-corner retroreflective sheeting and various improvements arestudied.

[0006] For example, Jungersen's U.S. Pat. No. 2,310,790 discloses aretroreflective sheeting constituted by arranging various shapes ofretroreflective elements on a thin sheeting and a method formanufacturing the sheeting. The triangular-pyramidal reflective elementsdisclosed in the above U.S. patent include a triangular-pyramidalreflective element in which the apex is located at the center of abottom-plane triangle and the optical axis does not tilt (that is, theoptical axis is vertical to the bottom plane) and a triangular-pyramidalreflective element in which the apex is not located at the center of abottom-plane triangle, and it is described in the U.S. patent toefficiently reflect light to an approaching automobile. Moreover, it isdescribed that the depth of a triangular-pyramidal reflective element iskept within {fraction (1/10)} in (2,540 μm). Furthermore, FIG. 15 in theU.S. patent shows a triangular-pyramidal reflective element whoseoptical axis has a tilt angle (θ) of approx. 6.5° obtained from theratio between the major side and the minor side of the bottom-planetriangle of the illustrated triangular-pyramidal reflective element.

[0007] However, the above Jungersen's U.S. patent does not specificallydisclose a very-small triangular-pyramidal reflective element disclosedby the present invention or does not describe or suggest a size of atriangular-pyramidal reflective element or a tilt angle the optical axisof the element necessary for superior observation angularity andentrance angularity.

[0008] Moreover, in the present specification, the expression “opticalaxis tilts in the plus (+) direction” denotes that the optical axistilts in the direction in which the difference between the distance (q)from the intersection (Q) of the optical axis of a triangular-pyramidalreflective element and the bottom plane (S_(x)-S_(x)′) of thetriangular-pyramidal reflective element up to the base edges (x,x, . . .) shared by the element pair {the distance (q) is equal to the distancefrom the intersection (Q) up to a plane (L_(x)-L_(x)) vertical to thebottom plane (S_(x)-S_(x)′) including the bottom edges (x,x, . . . )shared by the element pair} and the distance (p) from the vertical lineextended from the apex of the element to the bottom plane (S_(x)-S_(x)′)and the bottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . . ){the distance (p) is equal to the distance from the intersection (P) upto the vertical plane (L_(x)-L_(x))} becomes plus (+) as describedlater. On the contrary, when the optical axis tilts in the direction inwhich (q−p) becomes minus (−), the expression “optical axis tilts in thedirection for the optical axis to become minus (−)” is displayed.

[0009] Moreover, Stamm's U.S. Pat. No. 3,712,706 discloses aretroreflective sheeting in which the so-called triangular-pyramidalcube-corner retroreflective elements respectively having an equilateralbottom-plane triangle (therefore, the optical axis is vertical to abottom plane) are arranged on a thin sheeting so that bottom planes ofthe elements become the closest-packed state on a common plane. In theStamm's U.S. patent, means for improving the wide angularity inaccordance with the tilt of an optical axis is not described at all.

[0010] Furthermore, Hoopman's European Patent No. 137,736B1 discloses aretroreflective sheeting in which tilted triangular-pyramidalcube-corner retroreflective elements whose bottom-plane triangles areisosceles triangles are arranged on a common plane so that bottom planesof the elements become the closest-packed state. Moreover, it isdescribed that the optical axis of the triangular-pyramidal cube-cornerretroreflective element disclosed in the patent tilts in the minus (−)direction and its tilt angle approximately ranges between 7° and 13°.

[0011] Furthermore, Szczech's U.S. Pat. No. 5,138,488 similarlydiscloses a retroreflective sheeting in which tiltedtriangular-pyramidal cube-corner retroreflective elements whosebottom-plane triangles are isosceles triangles are arranged on a commonplane so that bottom planes of the elements become the closest-packedstate. In the U.S. patent, it is specified that the optical axis of apair of the triangular-pyramidal reflective elements tilts in thedirection of the side shared by the triangular-pyramidal reflectiveelement pair faced each other and the tilt angle ranges between approx.2° and 5° and each element has a size of 25 to 100 μm.

[0012] Moreover, in European Patent No. 548,280B1 corresponding to theabove patent, it is described that an optical axis tilts so that thedistance (p) between a plane including a side common to two elementswhich are paired and vertical to a common plan and the apex of anelement is not equal to the distance (q) between a point for the opticalaxis of an element to intersect with the common plane and the verticalplane and the tilt angle ranges between 2° and 5° and the height of anelement ranges between 25 and 100 μm.

[0013] As described above, in Szczech's European Patent No. 548,280B1,the tilt of an optical axis ranges between approx. 2° and 5° includingboth plus (+) and minus (−). However, embodiments of the above Szczech'sU.S. patent and European patent only specifically disclosetriangular-pyramidal reflective elements in which tilt angles of opticalaxes are −8.2°, −9.2°, and −4.3° and the height (h) of an element is87.5 μm.

[0014] The triangular-pyramidal cube-corner retroreflective elements inthe above-described conventionally publicly-known Jungersen's U.S. Pat.No. 2,310,790, Stamm's U.S. Pat. No. 3,712,706, Hoopman's EuropeanPatent No. 137,736B1, Szczech's U.S. Pat. No. 5,138,488, and EuropeanPatent No. 548,280B1 are common to each other in that bottom planes ofmany triangular-pyramidal reflective elements serving as cores ofentrance and reflection of light are present on the same plane.Moreover, every retroreflective sheeting constituted bytriangular-pyramidal reflective elements whose bottom planes are presenton the same plane is inferior in entrance angularity, that is, it has adisadvantage that the retroreflectivity suddenly decreases when anentrance angle of light to the triangular-pyramidal reflective elementincreases.

[0015] In general, the following are requested for atriangular-pyramidal cube-corner retroreflective sheeting as basicoptical characteristics: high brightness, that is, intensity (magnitude)of reflection brightness represented by the reflection brightness of thelight entering from the front of the sheeting, wide angularity, andthree performances such as observation angularity, entrance angularity,and rotation angularity about wide angularity.

[0016] As described above, every retroreflective sheeting constituted byconventionally publicly-known triangular-pyramidal cube-cornerretroreflective elements has a low entrance angularity and itsobservation angularity can not be satisfied in general. However, thepresent inventor et al. recently find that it is possible to improve theentrance angularity of a retroreflective sheeting constituted bytriangular-pyramidal reflective elements by making the depth (h′) of theelement from apexes (H₁ and H₂) of a face (face c) having one bottomside on the bottom plane (X-X′) of the triangular-pyramidal reflectiveelement {the depth is equal to the height of apexes (H₁ and H₂) from thebottom plane (X-X′)} substantially larger than the depth (h) of a plane(virtual plane Z-Z′) including base edges (z and w) of two faces (face aand face b) substantially perpendicularly crossing the face c of thetriangular-pyramidal reflective element from the apex of the plane. Theinvention of the present inventor et al. is announced in the officialgazette No. WO98/18028 internationally released on Apr. 30, 1998.

DISCLOSURE OF THE INVENTION

[0017] It is an object of the present invention to provide atriangular-pyramidal cube-corner retroreflective element (CC reflectiveelement) whose entrance angularity and rotation angularity areparticularly improved. According to the present invention, the aboveobject and advantage can be achieved by triangular-pyramidal cube-cornerretroreflective elements characterized in that the triangular-pyramidalcube-corner retroreflective elements protruded onto a common bottomplane (S_(x)-S_(x)′) share a base edge (x) on the bottom plane and arearranged on the bottom plane (S_(x)-S_(x)′) in the closest-packed stateso as to be faced each other, the bottom plane (S_(x)-S_(x)′) is acommon plane including many base edges (x,x, . . . ) shared by thetriangular-pyramidal reflective elements, two faced triangular-pyramidalreflective elements include the shared base edges (x,x, . . . ) on thebottom plane (S_(x)-S_(x)′) and form a pair of substantially-same-shapedelements faced each other so as to be substantially symmetric to planes(L_(x)-L_(x), L_(x)-L_(x), . . . ) vertical to the bottom plane(S_(x)-S_(x)′), and the optical axis of the triangular-pyramidalreflective element pair tilts so that the angle formed between theoptical axis and a vertical line extended from apexes (H₁ and H₂) of theelements to the bottom plane (S_(x)-S_(x)′) ranges between 0.5° and 1.5°in the direction in which the difference (q−p) between the distance (q)from the intersection (Q) of the optical axis and the bottom plane(S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared by the elementsand the distance (p) from the intersection (P) of the vertical line andthe bottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . . )shared by the element pair becomes plus (+) or minus (−).

[0018] In the case of the present invention, it is preferable to use atriangular-pyramidal cube-corner retroreflective element in which theoptical axis via apexes (H₁ and H₂) of the above triangular-pyramidalreflective elements tilts by 0.6° to 1.4° from a vertical line extendedfrom apexes (H₁ and H₂) of the above triangular-pyramidal reflectiveelements to the bottom plane (S_(x)-S_(x)′) in the direction for (q−p)to become plus (+) or minus (−).

[0019] In the case of the present invention, it is more preferable touse a triangular-pyramidal cube-corner retroreflective element in whichthe optical axis of the triangular-pyramidal reflective elements tiltsby 0.6° to 1.4° in the direction in which the difference (q−p) betweenthe distance (q) from the intersection (Q) of the optical axis and thebottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared bythe elements and the distance (p) from the intersection (P) of avertical line extended from apexes (H₁ and H₂) of the elements to thebottom plane (S_(x)-S_(x)′) and the bottom plane (S_(x)-S_(x)′) up tothe base edges (x,x, . . . ) shared by the elements becomes plus (+).

[0020] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which h, issubstantially larger than h_(y) and h_(z) when assuming the height froma bottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) sharedby two triangular-pyramidal reflective elements faced each other up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements ash_(x), the height from a bottom plane (S_(y)-S_(y)′) including the otherbase edges (y,y, . . . ) of the triangular-pyramidal reflective elementsup to apexes (H₁ and H₂) of the triangular-pyramidal reflective elementsas h_(y), and the height from a bottom plane (S_(z)-S_(z)′) includingthe still other base edges (z,z, . . . ) of the triangular-pyramidalreflective elements up to apexes (H₁ and H₂) of the triangular-pyramidalreflective elements as h_(z).

[0021] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which h_(y)and h_(z) are substantially equal to each other and h_(x) issubstantially larger than h_(y) and h_(z) when assuming the height froma bottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) sharedby two triangular-pyramidal reflective elements faced each other up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements ash_(x), the height from a bottom plane (S_(y)-S_(y)′) including the otherbase edges (y,y, . . . ) of the triangular-pyramidal reflective elementsup to apexes (H₁ and H₂) of the triangular-pyramidal reflective elementsas by, and the height from a bottom plane (S_(z)-S_(z)′) including thestill other base edges (z,z, . . . ) of the triangular-pyramidalreflective elements up to apexes (H₁ and H₂) of the triangular-pyramidalreflective elements as h_(z).

[0022] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which theoptical axis of the triangular-pyramidal reflective elements tilts by0.6° to 1.4° in the direction in which the difference (q−p) between thedistance (q) from the intersection (Q) of the optical axis and thebottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared bythe element pair and the distance (p) from the intersection (P) of avertical line extended from apexes (H₁ and H₂) of the elements to thebottom plane (S_(x)-S_(x)′) and the bottom plane (S_(x)-S_(x)′) up tothe base edges (x,x, . . . . ) shared by the elements becomes minus (−)and moreover, h_(y) and h_(z) are substantially equal to each other andh_(x) is substantially smaller than h_(y) and h_(z) when assuming theheight from a bottom plane (S_(x)-S_(x)′) including base edges (x,x, . .. ) shared by two triangular-pyramidal reflective elements faced eachother up to apexes (H₁ and H₂) of the triangular-pyramidal reflectiveelements as hx, the height from a bottom plane (S_(y)-S_(y)′) includingthe other base edges (y,y, . . . ) of the triangular-pyramidalreflective elements up to apexes (H₁ and H₂) of the triangular-pyramidalreflective elements as h_(y), and the height from a bottom plane(S_(z)-S_(z)′) including the still other base edges (z,z, . . . ) of thetriangular-pyramidal reflective elements up to apexes (H₁ and H₂) of thetriangular-pyramidal reflective elements as hz.

[0023] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which aninequality “1.03<h_(max)/h_(min)<1.3” is satisfied when at least two ofthe above h_(x), h_(y), and h_(z) are substantially different from eachother and the maximum one of the h_(x), h_(y), and h, is assumed ash_(max) and the minimum one of them is assumes as h_(min).

[0024] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which theabove h_(x), h_(y), and h_(z) respectively range between 50 and 500 μm(both included).

[0025] A still-more-preferable triangular-pyramidal cube-cornerretroreflective element of the present invention is atriangular-pyramidal cube-corner retroreflective element in which theabove h_(x), h_(y), and h_(z) respectively range between 60 and 200 μm.

[0026] In the case of the present invention, a triangular-pyramidalcube-corner retroreflective element is preferable in which at least oneprism-face angle formed when three lateral faces (faces a₁, b₁, and c₁)or (faces a₂, b₂, and C₂) of the triangular-pyramidal cube-cornerretroreflective element cross each other ranges between 89.5° and 90.5°and is slightly deviated from 90.000°.

[0027] In the case of the present invention, a triangular-pyramidalcube-corner retroreflective element is more preferable in which theabove triangular-pyramidal cube-corner retroreflective element issheet-like.

[0028] The present invention is described below more minutely.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1-A is a top view of a CC reflective element pair of thepresent invention in which base edges (x, y, and z) are present on thesame bottom plane and the optical axis tilts in the direction for theoptical axis to become plus (+) and FIG. 1-B is a sectional view of theCC reflective element pair;

[0030]FIG. 2 is a top view of a CC retroreflective body in which thebase edge (x) among three base edges (x), (y), and (z) is formed moredeeply than the other base edges (y) and (z) and a CC reflective elementpair of the present invention whose optical axis tilts in the plus (+)direction is set;

[0031]FIG. 3-A is a top view of a pair of CC reflective elements in theCC retroreflective body shown in FIG. 2 and FIG. 3-B is a sectional viewof the CC reflective element pair;

[0032]FIG. 4-A is a top view of a pair of CC reflective elements of thepresent invention in which the base edge (x) among three base edges (x),(y), and (z) is formed more shallowly than other base edges (y) and (z)and the optical axis tilts in the minus direction and FIG. 4-B is asectional view of the CC reflective element pair;

[0033]FIG. 5 is a sectional view showing a structure of atriangular-pyramidal cube-corner retroreflective sheeting in which CCreflective elements of the present invention are arranged;

[0034]FIG. 6 is an illustration showing entrance angularities oftriangular-pyramidal cube-corner retroreflective sheetings prepared forEmbodiments 1 and 2 and Comparative Examples 1 and 2; and

[0035]FIG. 7 is an illustration showing rotation angularities oftriangular-pyramidal cube-corner retroreflective sheetings prepared forEmbodiments 1 and 2 and Comparative Examples 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

[0036]FIG. 1-A and 1-B show a mode of a pair of triangular-pyramidalcube-corner retroreflective elements (CC reflective elements) R1 and R2of the present invention, in which FIG. 1-A is a top view of thereflective element pair and FIG. 1-B is a sectional view of thereflective elements R₁ and R₂ vertical to a common bottom plane(S_(x)-S_(x)′) including the points C₂, H₂, H₁, and C₁ in FIG. 1-A andbase edges (x,x, . . . ) shared by many paired reflective elements.

[0037] Moreover, in FIGS. 1-A and 1-B, H₁ and H₂ express apexes of cubecorners of the reflective elements R₁ and R₂, the reflective elements R₁and R₂ protrude onto the shared bottom plane (S_(x)-S_(x)′) and arefaced each other by sharing one base edge (x,x, . . . ) on the bottomplane (S_(x)-S_(x)′) to constitute a pair of CC reflective element pairsin a CC retroreflective bodies arranged on the bottom plane(S_(x)-S_(x)′) in the closest-packed state.

[0038] Moreover, the dotted line (H₁-P) in FIG. 1-B denotes a verticalline right-angled to the bottom plane (S_(x)-S_(x)′) from the apex H₁ ofthe reflective element R₁ and the dotted line (H₁-Q) denotes an opticalaxis passing through the apex H₁ of the reflective element R₁, andtherefore, the tilt of the reflective element R₁ is shown by θ.

[0039] Furthermore, the line x-x in FIG. 1-A shows one base edge (x,x, .. . ) shared by the pair of CC reflective elements R₁ and R₂ on onebottom plane (S_(x)-S_(x)′) shared by the CC reflective elements R₁ andR₂, the intersection with a vertical plane to the bottom plane(S_(x)-S_(x)′) from apexes of the reflective elements is shown by P, andthe intersection of an optical axis via the apex H₁ of the reflectiveelement R₁ and the bottom plane (S_(x)-S_(x)′) is shown by Q.

[0040] Furthermore, in FIG. 1-B, the line Lx-Lx denotes a plane verticalto one bottom plane (S_(x)-S_(x)′) shared by the reflective elements R₁and R₂ on one base edge (x) shared by the both elements R₁ and R₂.

[0041] The reflective elements R₁ and R₂ form asubstantially-same-shaped element pair faced each other so as to besubstantially symmetric to the plane (L_(x)-L_(x)) vertical to thebottom plane (S_(x)-S_(x)′) and the same is applied to FIGS. 3-A and 3-Band FIGS. 4-A and 4-B to be described later.

[0042] In the present invention, the fact that the tilt {tilt from theshared bottom plane (S_(x)-S_(x)′)} of the optical axis (H₁-Q) of thereflective element R₁ is plus (+) denotes that (q−p) is plus (+), thefact that the tilt of it is minus denotes that (q−p) is minus, and thefact that (q−p) is 0 denotes that the optical axis is vertical to thecommon bottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) ofmany paired reflective elements.

[0043] The present invention is characterized in that atriangular-pyramidal cube-corner retroreflective element has an opticalaxis tilting by 0.5 to 1.5° and particularly, a triangular-pyramidalcube-corner retroreflective element having an optical axis tilting by0.6 to 1.4° is preferable. In FIG. 1-B, the tilt θ of the optical axisis emphasized into approx. 5° instead of a tilt of 0.5 to 1.5° of a CCreflective element of the present invention so that a tilt state can beeasily understood. The same is applied to FIGS. 3-B and 4-B to bedescribed later.

[0044] In the case of the present invention, though it is allowed thatthe tilt of an optical axis {that is, the above (q−p)} is a plus (+)direction or a minus direction (−), it is preferable that the opticalaxis tilts in the plus (+) direction.

[0045] Therefore, in the case of the present invention, atriangular-pyramidal cube-corner retroreflective element is preferablein which the optical axis of the triangular-pyramidal reflectiveelements tilts by 0.6 to 1.4° in the direction in which the difference(q−p) between the distance (q) from the intersection (Q) of the opticalaxis and the bottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . .. ) shared by the element pair and the distance (p) from theintersection (P) of a vertical line extended from apexes (H1 and H2) ofthe elements to the bottom plane (S_(x)-S_(x)′) and the bottom plane(S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared by the elementpair becomes plus (+).

[0046] According to the study by the present inventor et al., it isfound that the above reflective element of the present invention isparticularly superior not only in entrance angularity but also inrotation angularity.

[0047]FIG. 2 and FIGS. 3-A and 3-B show another mode of a pair oftriangular-pyramidal cube-corner retroreflective elements (CC reflectiveelements) R₁ and R₂ of the present invention, in which FIG. 2 is a topview of a CC retroreflective body in which reflective elements arearranged, FIG. 3-A is a top view of a pair of elements of the CCretroreflective body shown in FIG. 3, and FIG. 3-B is a sectional viewof the reflective elements R₁ and R₂ vertical to a common bottom plane(S_(x)-S_(x)′) including base edges (x,x, . . . ) including points C₂,H₂, H₁, and C₁ in FIG. 3-A and shared by many paired reflectiveelements.

[0048] In FIG. 2, a₁, b₁, and c₁ and a₂, b₂, and C₂ denote lateral facesof many arranged reflective elements such as the reflective elements R₁and R₂ in FIG. 2-A, x denotes an adjacent reflective element such as abase edge shared by lateral faces (faces c₁ and C₂) of R₁ and R₂, ydenotes a base edge shared by lateral faces (faces b₁ and b₂) of anadjacent reflective element separate from R₁ and R₂, and z denotes abase edge shared by lateral faces (a₁ and a₂) of a still anotheradjacent reflective element. Reflective elements adjacent by sharing theabove base edges (x,x, . . . ) include the above base edges (x,x, . . .), form a substantially-same-shaped element pair faced each other so asto be substantially symmetric to planes (L_(x)-_(x), L_(x)-L_(x), . . .) vertical to the bottom plane (S_(x)-S_(x)′), and are arranged on thebottom plane (S_(x)-S_(x)′) in the closest-packed state.

[0049] In FIGS. 3-A and 3-B, a dotted line S_(x)-S_(x)′ denotes a bottomplane including the above base edges (x,x, . . . ) and a dotted lineS_(y)-S_(y)′ denotes a bottom plane including the above base edges (y,y,. . . ), and a dotted line S_(z)-S_(z)′ denotes a bottom plane includingthe above base edges (z,z, . . . ). Symbol hx denotes the height from abottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) shared bytwo triangular-pyramidal reflective elements faced each other up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements,h_(y) denotes the height from a bottom plane (S_(x)-S_(x)′) includingother base edges (y,y, . . . ) up to apexes (H₁ and H₂) of thetriangular-pyramidal reflective elements, and hz denotes the height froma bottom plane (S_(x)-S_(x)′) including still other base edges (z,z, . .. ) of the triangular-pyramidal reflective elements up to apexes (H₁ andH₂) of the triangular-pyramidal reflective elements.

[0050] In the case of this mode, the bottom plane (S_(x)-S_(x)′) ispresent at a position lower than the bottom planes (S_(y)-S_(y)′) and(S_(z)-S_(z)′) and the bottom plane (S_(y)-S_(y)′) and the bottom plane(S_(x)-S_(x)′) are on the sample plane. That is, h_(x) is larger thanh_(y) and h_(z) and h_(y) and h_(z) are equal to each other.

[0051] Therefore, the lateral faces (a₁ and a₂) and (b₁ and b₂) arerespectively formed into a shape slightly cut out by the lateral faces(c₁ and C₂) and the lateral faces (a₁ and a₂) and (b₁ and b₂) arerespectively formed into a quadrangle, and the lateral faces (c₁ and c₂)are respectively formed into a pentagon.

[0052] As shown in FIG. 3-B, similarly to the case of the CC reflectiveelements shown in FIG. 1, optical axes (H₁-Q and H₂-Q) of the CCreflective element pair tilt by 0.5 to 1.5° in the direction in whichthe difference (q−p) between the distance (p) from the intersection (P)of a vertical line extended from apexes (H₁ and H₂) of the CC reflectiveelements up to the bottom plane (S_(x)-S_(x)′) and the distance (q) fromthe intersection (Q) of the optical axes and the bottom plane(S_(x)-S_(x)′) up to a base edge (x) shared by the CC reflective elementpair becomes plus (+).

[0053] FIGS. 4-A and 4-B show still another mode of a pair oftriangular-pyramidal cube-corner retroreflective elements (CC reflectiveelements) of the present invention, in which FIG. 4-A is a top view ofthe reflective element and FIG. 4-B is a sectional view of reflectiveelements R₂ and R₁ vertical to a common bottom plane (S_(x)-S_(x)′)including points C₂, H₂, H₁, and C₁ in FIG. 4A and base edges (x,x, . .. ) shared by may paired reflective elements.

[0054] In the case of this mode, the bottom plane (S_(x)-S_(x)′)including the base edges (x,x, . . . ) is present at a position higherthan a bottom plane (S_(y)-S_(y)′) including the base edges (y,y, . . .) and a bottom plane (S_(z)-S_(z)′) including the base edges (z,z, . . .) and the bottom planes (S_(y)-S_(y)′) and (S_(z)-S_(z)′) are present onthe same plane. That is, h_(x) is smaller than h_(y) and h_(z) and h_(y)and h_(z) are equal to each other.

[0055] Therefore, lateral faces (c₁ and C₂) are respectively formed intoa shape slightly cut out by lateral faces (a₁ and a₂) and (b₁ and b₂),lateral faces (a₁ and a₂) and (b₁ and b₂) are respectively formed into aquadrangle, and lateral faces (c₁ and C₂) are respectively formed into atriangle.

[0056] Moreover, as shown in FIG. 4-B, optical axes (H₁-Q and H₂-Q) ofthe CC reflective element pair tilts by 0.5 to 1.5° in the direction inwhich the difference (q−p) between the distance (p) from theintersection (P) of a vertical line extended from apexes (H₁ and H₂) ofthe CC reflective elements to the bottom plane (S_(x)-S_(x)′) and thedistance (q) from the intersection (Q) of the optical axes and thebottom plane (S_(x)-S_(x)′) up to a base edge (x) shared by the CCreflective element pair becomes minus (−).

[0057] In the case of a pair of CC reflective elements of the presentinvention, it is allowed that h_(x), h_(y), and h_(z) are equal to ordifferent from each other when assuming the height from a bottom plane(S_(x)-S_(x)′) including base edges (x,x, . . . ) shared by the CCreflective elements faced each other up to apexes (H₁ and H₂) of the CCreflective element pair as h_(x), the height from a bottom plane(S_(y)-S_(y)′) including other base edges (y,y, . . . ) of the CCreflective elements up to apexes (H₁ and H₂) of the CC reflectiveelements as h_(y), and the height from a bottom plane (S_(z)-S_(z)′)including still other base edges (z,z, . . . ) of the CC reflectiveelements up to apexes (H₁ and H₂) of the CC reflective elements ash_(z). From the viewpoint of entrance angularity, however, it ispreferable that h_(x) is substantially larger than h_(y) and h_(z) whenthe optical axis tilts in the direction for (q−p) to become plus (+).Moreover, when the optical tilts in the direction for (q−p) to becomeminus (−), it is preferable that h_(x) is substantially smaller thanh_(y) and h_(z).

[0058] Furthermore, when at least two of the above h_(x), h_(y), andh_(z) are substantially different from each other and the maximum one ofthe h_(x), h_(y), and h_(z) is assumed as hoax and the minimum one ofthem is assumed as h_(min), it is preferable that an inequality“1.03<h_(max)<h_(min)<1.3” is satisfied and it is more preferable thatan inequality “1.05<h_(max)/h_(min)<1.2” is satisfied.

[0059] In the case of a CC reflective element satisfying the value ofthe above h_(max)/h_(min), it is possible to increase the quantity oflight to be three-face-reflected and retroreflected because it ispossible to almost equalize the area of three lateral faces (c₁ and C₂),that of lateral faces (a₁ and a₂), and that of lateral faces (b₁ and b₂)each other.

[0060] It is recommended that heights h_(x), h_(y), and h_(z) of theabove CC reflective element respectively preferably range between 50 and500 μm and more preferably range between 60 and 200 μm. When any one ofthe heights h_(x), h_(y), and h_(z) is less than 50 μm, the size of areflective element becomes too small. Therefore, retroreflected light isextremely diverged due to the diffraction effect decided in accordancewith the planar opening area of the reflective element and the frontreflectivity is deteriorated. Moreover, it is not preferable that anyone of the heights h_(x), h_(y), and h_(z) exceeds 500 μm because thethickness of a sheeting becomes excessive and a soft sheeting cannot beeasily obtained.

[0061] Furthermore, three prism face angles formed by the fact thatthree lateral faces (a₁, b₁, and c₁) or (a₂, b₂, and C₂) serving asprism faces of a CC reflective element of the present invention crosseach other substantially form right angles. However, it is preferablethat the right angle is not always a strict right angle (90.000°) butthe right angle is a right angle slightly deviated from a true rightangle. By providing a very slight angular deviation to the prism faceangles, it is possible to properly diverge the right reflected from anobtained CC reflective element. However, when the angular deviation istoo large, the light reflected from the obtained CC reflective elementexcessively diverges and the retroreflectivity is deteriorated.Therefore, it is preferable that at least one prism face angle formedwhen these three lateral faces (a₁, b₁, and c₁) or (a₂, b₂, and C₂)cross each other ranges between 89.5° and 90.5°, more preferable thatthe prism face angle ranges between 89.7° and 90.3°, and the angle isslightly deviated from 90.000°.

[0062] A triangular-pyramidal cube-corner retroreflective element (CCreflective element) of the present invention is used as a CCretroreflective body by collecting many CC reflective elements.Moreover, it is allowed to form many CC reflective elements into atriangular-pyramidal cube-corner retroreflective sheeting and set itonto a purposed object such as a vehicle or traffic sign or directlyform many CC reflective elements on an object. Though the usage of theCC reflective elements is not restricted, they are generally used byforming them into a sheeting.

[0063] Then, a mode of a preferable structure of a triangular-pyramidalcube-corner retroreflective sheeting on which CC reflective elements ofthe present invention are arranged is described below by referring toFIG. 5 showing a sectional view of the mode.

[0064] In FIG. 5, symbol 1 denotes a reflective element layer on whichtriangular-pyramidal reflective elements (R₁ and R₂) are arranged in theclosest-packed state, 2 denotes a holding-body layer for holding CCreflective elements, and 10 denotes a light entrance direction. Thoughthe reflective element layer (1) and holding-body layer (2) aregenerally united into one body, it is also allowed to laminate separatelayers. In accordance with an applying purpose or applying environmentof a retroreflective sheeting of the present invention, it is possibleto add a surface protective layer (4), a printing layer (5) fortransferring information to an observer or coloring the sheeting, abinder layer (6) for realizing an airtight structure for preventingmoisture from entering the back of a CC reflective element, a supportlayer (7) for supporting the binder layer (6), and an adhesive layer (8)for attaching the retroreflective sheeting to other structure and arelease-agent layer (9) such as a release film.

[0065] It is possible to use the resin same as that used for thereflective element layer (1) for the surface protective layer (4).However, it is also allowed to mix an ultraviolet absorbent, a lightstabilizer, and an antioxidant in the surface protective layer (4)independently or by combining them in order to improve the weatherresistance. Moreover, it is possible to make the layer (4) containvarious organic pigments, inorganic pigments, and dyes as coloringagents.

[0066] It is generally possible to set the printing layer (5) betweenthe surface protective layer (4) and the holding-material layer (2) oron the surface protective layer (4) or the reflective face of the CCreflective element (1) by a means such as gravure printing, screenprinting, or ink-jet printing.

[0067] It is possible to use any material for the reflective elementlayer (1) and holding-body layer (2) as long as the material meets theflexibility that is one of the objects of the present invention.However, it is preferable to use a material having optical transparencyand uniformity. The following are materials which can be used for thepresent invention: polyolefin resins such as a polycarbonate resin,vinyl chloride resin, (meth)acrylic resin, epoxy resin, polystyreneresin, polyester resin, fluorocarbon resin, polyethylene resin, andpolypropylene resin and cellulose-based resins, and polyurethane resins.

[0068] In the case of the reflective element layer (1) of the presentinvention, it is general to set the air layer (3) to the back of acube-corner retroreflective element in order to increase a criticalangle for satisfying the internal total reflection condition. To preventtroubles such as decrease of a critical angle due to incoming ofmoisture and corrosion of a metallic layer, it is preferable that-thereflective element layer (1) and support layer (7) are sealed by thebinder layer (6). As the sealing method, it is possible to use themethods disclosed in U.S. Pat. Nos. 3,190,178 and 4,025,159 and JapaneseUtility Model No. 28669/1975. The following resins are sued for thebinder layer (6): (meth)acrylic resin, polyester resin, alkyd resin, andepoxy resin. As joining methods, it is possible to properly usepublicly-known thermal-fusion-bonding-resin joining method,thermosetting-resin joining method, ultraviolet-curing-resin joiningmethod, and electron-beam-curing-resin joining method.

[0069] It is possible to apply the binder layer (6) to the entiresurface of the support layer (7) or selectively set the layer (6) to thejoint with a retroreflective-element layer in accordance with theprinting method or the like.

[0070] As a material for forming the support layer (7), it is possibleto use a resin for forming a retroreflective-element layer, a resincapable of forming a general film, fiber, cloth, and a metallic foil orplate made of stainless steel or aluminum individually or by combiningthem.

[0071] It is possible to properly select a publicly-known material forthe adhesive layer (8) and the release-agent layer (9) for the adhesiveused to attach a retroreflective sheeting of the present invention to awood plate, glass plate, or plastic plate.

[0072] A triangular-pyramidal cube-corner retroreflective sheeting onwhich CC reflective elements of the present invention are arranged isgenerally used so that the light 10 enters from the upper portion of thesurface protective layer (1) by turning the CC reflective elements shownin FIG. 5 upside down. Therefore, the sheeting can be manufactured byusing a cube-corner-molding die in which shapes of the above-describedCC reflective elements are arranged on a metallic belt as reversedconcave shapes in the closest-packed state, thermally pressing a propersoft resin sheeting superior in optical transparency and uniformity tobe described later against the molding die, and inversely transferringthe shapes of the die to the resin sheeting.

[0073] A typical manufacturing method of the above cube-corner-moldingdie is described in the above Stamm's patent in detail and it ispossible to use a method conforming to the above method for the presentinvention.

[0074] Specifically, a microprism mother die in which convex finetriangular pyramids are arranged in the closest-packed state is formedby using a carbide cutting tool (such as diamond cutting tool ortungsten-carbide cutting tool) having a tip angle of approx. 67.5 to73.5°, deciding two-directional (y-direction and z-direction in FIG. 2)repetitive pitches, groove depths (h_(y) and h_(z)), and mutual crossingangles in accordance with the shape of a purposed CC reflective element,cutting parallel grooves respectively having a V-shaped cross section,and then cutting V-shaped parallel grooves in the third direction (xdirection) at a repetitive pitch (repetitive pitch of the line x in FIG.2) passing through the intersection of the formed y-directional grooveand z-directional groove and bisecting the supplementary angle of thecrossing angle of these two directions (in this case, an acute angle isreferred to as “crossing angle”).

[0075] In the case of a preferred mode of the present invention,y-directional and z-directional repetitive pitches range between approx.210.3 and 214.2 μm, groove depths (h_(y) and h_(x)) range betweenapprox. 50 and 500 μm, a mutual crossing angle ranges between approx.59.1 and 60.9°, an x-directional groove depth ranges between approx.208.6 and 216.4 μm, and a depth (h_(x)) ranges between approx. 38.5 and650 μm.

[0076] These x-, y-, and z-directional grooves are cut so that eachgroove cross section is generally formed into an isosceles triangle.However, it is also possible to cut at least one-directional grooveamong these three directional grooves so that the cross section of thegroove is slightly deviated from an isosceles triangle according tonecessity. Specifically, one of the following methods can be used: amethod of cutting a groove by a cutting tool whose front-end shape isasymmetric to right and left or cutting a groove while slightly tiltinga cutting tool whose front-end shape is symmetric to right and left.Thus, it is possible to provide a very-small angular deviation from theright angle (90°) for at least one of the prism face angles of threelateral faces (a₁, b₁, and c₁) or (a₂, b₂, and C₂) of a CC reflectiveelement obtained by slightly deviating the cross section of a groovefrom an isosceles triangle and thereby, properly diverge the lightreflected from the CC reflective element from a complete retroreflectivedirection.

[0077] As a base material which can be preferably used to form the abovemother die, it is preferable to use a metallic material having a Vickershardness (JIS Z 2244) of 300 or more, particularly preferable to use ametallic material having a Vickers hardness of 380 or more.Specifically, any one of amorphous copper, electrodeposition nickel, andaluminum can be used. As an alloy-based material, it is possible to useany one of copper-zinc alloy (brass), copper-tin-zinc alloy,nickel-cobalt alloy, nickel-zinc alloy, and aluminum alloy.

[0078] Moreover, as the above base material, it is also possible to usea synthetic resin material. Because a trouble that a material issoftened when cut and thereby it is difficult to cut the material at ahigh accuracy does not easily occur, it is preferable to use a materialmade of a synthetic resin having a glass transition point of 150° C. orhigher, particularly 200° C. or higher and a Rockwell hardness (JIS Z2245) of 70 or more, particularly 75 or more. Specifically, it ispossible to use any one of polyethylene-terephthalate-based resin,polybutylene-terephthalate-based resin, polycarbonate-based resin,polymethacrylate-based resin, polyimide-based resin, polyacrylate-basedresin, polyether-sulfone-based resin, polyether-imide-based resin, andcellulose-triacetate-based resin.

[0079] A flat plate can be formed by any one of the above syntheticresins in accordance with the normal resin-forming method such as theextraction-molding method, calender-molding method, or solution castmethod and moreover, heating or drawing can be performed according tonecessity. It is possible to apply the preparatory conduction treatmentto the plane of the flat plate thus formed in order to simply theconduction treatment and/or electroforming when forming anelectroforming die from a prism mother die manufactured in accordancewith the above method.

[0080] The preparatory conduction treatment includes the vacuumevaporation method for evaporating metals such as gold, silver, copper,aluminum, zinc, chromium, nickel, and selenium, the anode-sputteringmethod for using the above metals, and the electroless plating methodusing copper and nickel. Moreover, it is allowed to blend conductivepowder such as carbon black or organic metallic salt into a syntheticresin so that a flat plate has conductivity.

[0081] Then, the surface of the obtained microprism mother die iselectroformed and a metallic film is formed on the surface. By removingthe metallic film from the surface of the mother die, it is possible toform a metallic die used to mold a triangular-pyramidal cube-cornerretroreflective sheeting of the present invention.

[0082] In the case of a metallic microprism mother die, immediatelyafter cleaning the surface of the die, it is possible to electroform thesurface. In the case of a synthetic-resin microprism mother die, it isnecessary to apply the conduction treatment for providing conductivityto the surface of the prism of the mother die before electroforming thesurface. As the conduction treatment, it is possible to use any one ofthe silver mirror treatment, electroless plating, vacuum evaporation,and cathode sputtering.

[0083] As the above silver mirror treatment, it is possible to use amethod of cleaning the surface of a mother die formed by theabove-described method with an alkaline detergent to remove thecontamination such as oil component or the like, then activating thesurface with a surface activator such as tannic acid, and then finishingthe surface like a silver mirror. To finish the surface like a silvermirror, either of the following methods can be used: the spraying methodusing a double-cylinder-type nozzle gun for a silver-nitride aqueoussolution and a reducing-agent (glucose or glyoxal) aqueous solution andthe method of immersing a mother die in a mixed solution of thesilver-nitride aqueous solution and reducing-agent aqueous solution.Moreover, it preferable that the thickness of a sliver-mirror film issmaller as long as the conductivity under electroforming is satisfiedand for example, a thickness of 0.1 μm or less can be used.

[0084] Electroless plating uses copper or nickel. An electroless platingsolution can use nickel sulfate or nickel chloride as a water-solublemetallic salt of nickel. A solution obtained by adding a solution mainlycontaining citrate or malate to the water-soluble metallic salt ofnickel as a complexing agent and sodium hypophosphite or amine Volan tothe metallic salt as a reducing agent is used as a plating solution.

[0085] The vacuum evaporation can be performed by cleaning the surfaceof a mother die similarly to the case of the sliver mirror treatment,then putting the die in a vacuum system, heating and vaporizing a metalsuch as gold, silver, copper, aluminum, zinc, nickel, chromium, orselenium and precipitating it on the cooled mother-die surface, andforming a conductive film. Moreover, the cathode sputtering can beformed by putting a mother die treated same as the case of the vacuumevaporation in a vacuum system in which a flat cathode plate capable ofmounting a desired metallic foil and an anode table made of a metal suchas aluminum or iron for mounting a material to be treated are set andputting the mother die on the anode table, setting a metallic foil sameas one used for the case of the vacuum evaporation to a cathode andelectrifying the foil to cause glow discharge, making a cation flowgenerated by the glow discharge collide with the metallic foil on thecathode and thereby evaporating metallic atoms or particulates, andprecipitating the atoms or particulates on the surface of the mother dieto form a conductive film. A thickness of 300 Å can be used as thethickness of conductive films formed by theses methods.

[0086] To form a smooth and uniform electroformed layer on asynthetic-resin mother die through electroforming, it is necessary touniformly apply the above conduction treatment to the entire surface ofthe mother die. When the conduction treatment is ununiformly performed,a trouble may occur that the smoothness of the surface of anelectroformed layer having a low conductivity is deteriorated and noelectroformed layer is formed but a defect is formed.

[0087] To avoid the above trouble, it is possible to use a method forimproving the wetting of a silver-mirror solution by treating atreatment face with alcohol immediately before starting the silvermirror treatment. However, because a synthetic-resin mother die formedin the present invention has a very-deep concave portion forming anacute angle, improvement of wetting tends to be insufficient. A troubleof a conductive film due to the concave portion also easily occurs inthe evaporation.

[0088] To uniform the surface of an electroformed layer obtained throughelectroforming, activation is frequency performed. As the activation, itis possible to use a method of immersing the electroformed layer in a10-wt % sulfamic-acid aqueous solution.

[0089] When electroforming a synthetic-resin mother die silver-mirrortreated, a silver layer is integrated with an electroformed layer andeasily removed from the synthetic-resin mother die. However, whenforming a conductive film made of nickel or the like through electrolessplating or cathode sputtering, it may be difficult to remove anelectroformed layer from a synthetic-resin layer because the surface ofthe synthetic-resin layer closely contacts with the conductive film. Inthis case, it is preferable to apply the so-called removal treatmentsuch as chromating onto a conductive-film layer before electroforming.In this case, the conductive-film layer remains on a synthetic-resinlayer after removed.

[0090] In the case of a synthetic-resin prism mother die on whosesurface a conductive-film layer is formed, the above various treatmentsare applied and then an electroformed layer is formed on theconductive-film layer by electroforming. Moreover, in the case of ametallic prism mother die, the surface is cleaned according to necessityas described above and then, an electroformed layer is directly formedon the metal.

[0091] Electroforming is generally performed in a 60-wt % aqueoussolution of nickel sulfamate at 40° C. under a current condition ofapprox. 10A/dm². By setting an electroformed-layer forming rate to, forexample, 48 hr/mm or less, a uniform electroformed layer can be easilyobtained. However, at a forming rate of 48 hr/mm or higher, a troubleeasily occurs that the smoothness of the surface of a layer isdeteriorated or a defect is formed.

[0092] Moreover, in the case of electroforming, it is possible toelectroform a nickel-cobalt alloy to which a component such as cobalt isadded in order to improve the surface abrassiveness of a die. By adding10 to 15 wt % of cobalt, it is possible to increase the Vickers hardnessHv of an obtained electroformed layer up to 300 to 400, it is possibleto mold a synthetic resin by using an obtained electroforming die andimprove the durability of the die when manufacturing atriangular-pyramidal cube-corner retroreflective sheeting of the presentinvention.

[0093] A first-generation electroforming die thus formed from a prismmother die can be repeatedly used as an electroforming master used tofurther form a second-generation electroforming die. Therefore, it ispossible to form several electroforming dies by one prism mother die.

[0094] It is possible to use a plurality of formed electroforming diesby precisely cutting them and then, combining and joining them up to afinal die size for molding a microprism sheeting using a syntheticresin. To join the electroforming dies, it is possible to use one of amethod of merely bringing cut ends face to face and a method of weldingcombined joints through electron-beam welding, YAG laser welding, orcarbonic-acid-gas laser welding.

[0095] Combined electroforming dies are used to mold a synthetic resinas synthetic-resin molding dies. The synthetic-resin molding method canuse compression molding or extrusion molding. The compression moldingcan be performed by inserting a formed thin-wall nickel electroformingdie, a synthetic-resin sheeting having a predetermined thickness, and asilicon-rubber sheeting having a thickness of approx. 5 mm serving as acushion material into a compression-molding press heated up to apredetermined temperature, then preheating them for 30 sec at a pressureof 10-20% of a molding pressure, and then heating and pressing them forapprox. 2 min at 180-250° C. and 10-30 kg/cm². Thereafter, it ispossible to obtain a molded product by cooling the above components upto room temperature while pressing them and then releasing the pressure.

[0096] Moreover, it is possible to obtain a continuous sheet-likeproduct by joining a thin-wall electroforming die having a thickness ofapprox. 0.5 mm formed by the above method, forming an endless belt dieby the above welding method, setting and rotating the belt die on a pairof rollers constituted by a heating roller and a cooling roller,supplying melted synthetic resin to the belt die on the heating rollerin the form of a sheet, pressure-molding the synthetic resin with one ormore silicon rollers, then cooling the synthetic resin to the glasstransition point temperature or lower with the cooling roller, andremoving the resin from the belt die.

[0097] The present invention is more minutely described below inaccordance with embodiments.

[0098] Embodiment 1:

[0099] Parallel grooves respectively having a V-shaped cross section areformed on a 100 mm-square brass plate whose surface is flatly ground arecut in accordance with the fly cutting method at a repetitive pattern byusing a diamond cutting tool having a tip angle of 68.53° so that in thefirst direction (y-direction in FIG. 2) and the second direction(z-direction in FIG. 2), first- and second-directional pitches are210.88 μm, groove depths (h_(y) and h_(x)) are 100 μm, and a crossingangle between the first and second directions is 58.76°.

[0100] Thereafter, the V-shaped parallel grooves are cut by using adiamond cutting tool having a tip angle of 71.52° so that a repetitivepitch (repetitive pitch of the line x in FIG. 1) is 214.92 μm, thegroove depth (h_(x)) is 100 μm, and crossing angles between the first,second, and third directions are 60.62° to form a mother die in whichmany convex triangular-pyramidal cube corners having the height (h_(x))of 100 μm from a virtual plane (S_(x)-S_(x)′) of a triangular-pyramidalreflective element are arranged in the closest-packed state. Theoptical-axis tilt angle θ of the triangular-pyramidal reflective elementis +1° and prism face angles of three faces constituting a triangularpyramid are 90°.

[0101] A concave cube-corner-molding die which is made of nickel andwhose shape is reversed is formed by using the above brass mother die inaccordance with the electroforming method. By using the molding die, apolycarbonate-resin sheeting (Iupilon E2000 made by MitsubishiEngineering-Plastics Corp.) having a thickness of 200 μm iscompression-molded at a molding temperature of 200° C. and a moldingpressure of 50 kg/cm², then, cooled up to 30° C. under the pressure, andthen taken out to form a triangular-pyramidal cube-cornerretroreflective sheeting made of polycarbonate resin in which cubecorners whose support layer has a thickness of approx. 250 μm and h_(x),h_(x), and hz are all equal to 100 μm are arranged in the closest-packedstate.

[0102] Embodiment 2:

[0103] Grooves respectively having a V-shaped cross section are cut on a100 mm-square brass plate whose surface is flatly ground at a repetitivepattern in accordance with the fly cutting method by using a diamondcutting tool having a tip angle of 68.53° in the first direction(y-direction) and the second direction (z-direction) and a tip angle of7.152° in the third direction (x-direction) so that repetitive pitchesin the first and second directions are 210.88 μm, cut-groove depths(h_(y) and h_(z)) are 100 μm, the crossing angle between the first andsecond directions is 58.76°, the repetitive pitch in the third directionis 214.92 μm, and a cut-groove depth (h_(x)) is 110 μm to form a motherdie in which many convex triangular-pyramidal cube corners having theheight (h_(y)) of 100 μm from a virtual plane (S_(y)-S_(y)′) of atriangular-pyramidal reflective element are arranged in theclosest-packed state. The optical-axis tilt angle θ of thetriangular-pyramidal reflective element is +1°. Moreover, the value ofhmax/hmin is 110/100=1.100.

[0104] Then, a concave cube-corner molding die made of nickel is formedsimilarly to the case of Embodiment 1 and a polycarbonate-resin sheetingsame as that of Embodiment 1 is formed by using the die under the sameconditions as the case of Embodiment 1 by using the die to form atriangular-pyramidal cube-corner retroreflective sheeting made ofpolycarbonate resin on whose surface cube corners respectively having asupport-layer thickness of approx. 250 μm and having h_(y) and h_(z) of100 μm respectively and h_(x) of 110 μm are arranged in theclosest-packed state.

[0105] Embodiment 3:

[0106] Parallel grooves respectively having a V-shaped cross section arecut in the first and second directions at a repetitive pattern inaccordance with the fly cutting method by using a diamond cutting toolhaving a tip angle of 72.53° so that repetitive pitches in the first andsecond directions are 213.5 μm, groove depths (h_(y) and h_(z)) are 100μm, and the crossing angle between the first and second directions is61.21°.

[0107] Thereafter, V-shaped parallel grooves are cut in the thirddirection (x-direction) by using a diamond cutting tool having a tipangle of 69.52° so that the repetitive pitch (repetitive pitch of theline x in FIG. 1) is 209.67 μm, the groove depth (h_(x)) is 100 μm, andthe crossing angle between the first and second directions is 59.40° toform a mother die in which many convex triangular-pyramidal cube cornershaving the height (h_(x)) of 100 μm from a virtual plane (S_(x)-S_(x)′)of a triangular-pyramidal reflective element are arranged in theclosest-packed state on a brass plate. The optical-axis tilt angle θ ofthe triangular-pyramidal reflective element is −1° and three prism facesconstituting a triangle are all 90°.

[0108] A cube-corner-molding die made of nickel is formed similarly tothe case of Embodiment 1 and a polycarbonate-resin sheeting same as thatof Embodiment 1 is compression-molded by using the die under the samemolding conditions as the case of Embodiment 1 to form atriangular-pyramidal cube-corner retroreflective sheeting made ofpolycarbonate resin in which cube corners having a support-layerthickness of approx. 250 μm and having h_(x)=h_(z)=h_(z) of 100 μm arearranged in the closest-packed state.

[0109] Embodiment 4:

[0110] Grooves respectively having a V-shaped cross section are cut on a100 mm-square brass plate whose surface is flatly ground at a repetitivepattern in accordance with the fly cutting method by using a diamondcutting tool having a tip angle of 72.53° in the first direction(y-direction) and second direction (z-direction) and a tip angle of69.52 in the third direction (x-direction) so that repetitive pitches inthe first and second directions are 213.50 μm, groove depths (h_(y) andh_(z)) are 100 μm, and the crossing angle between the first and seconddirections is 61.21°, the repetitive pitch in the third direction is209.67 μm, and the cut groove depth (h_(x)) is 90 μm to form a motherdie in which many convex triangular-pyramidal cube corners having theheight of 100 μm from a virtual plane (S_(y)-S_(y)′) of atriangular-pyramidal reflective element are arranged in theclosest-packed state on the brass plate. The optical-axis tilt angle θof the triangular-pyramidal reflective element is −1°. Moreover, thevalue of h_(max)/h_(min) is 100/90=1.11.

[0111] Then, a concave cube-corner-molding die made of nickel is formedsimilarly to the case of Embodiment 1 and a polycarbonate-resin sheetingsame as that of Embodiment 1 is compression-molded by using the dieunder the same molding conditions as the case of Embodiment 1 to form atriangular-pyramidal cube-corner retroreflective sheeting made ofpolycarbonate resin in which cube corners having a support-layerthickness of approx. 250 μm and having h_(y)=y_(z)=100 μm and h_(x)=90μm are arranged in the closest-packed state.

COMPARATIVE EXAMPLE 1:

[0112] Grooves respectively having a V-shaped cross section are cut on a100 mm-square brass plate whose surface is flatly ground at a repetitivepattern in accordance with the fly cutting method by using a diamondcutting tool having a tip angle of 70.53° in the first direction(y-direction), second direction (z-direction), and third direction(x-direction) so that repetitive pitches in the first, second, and thirddirections are 212.13 μm and the crossing angle between the first andsecond directions is 60.00° to form a mother die in which many convextriangular-pyramidal cube corners whose cube-corner retroreflectiveelements have a height of 100 μm are arranged on the brass plate in theclosest-packed state. The optical-axis tilt angle θ of the reflectiveelements is 0° and three prism faces constituting a triangular pyramidare all 90°.

[0113] A triangular-pyramidal cube-corner retroreflective sheeting madeof polycarbonate resin is formed in accordance with the same method asthe case of Embodiment 1.

COMPARATIVE EXAMPLE 2:

[0114] Grooves respectively having a V-shaped cross section are cut on a100 mm-square brass plate whose surface is flatly ground at a repetitivepattern in accordance with the fly cutting method by using a diamondcutting tool having a tip angle of 62.53° in the first direction(y-direction) and the second direction (z-direction) and a tip angle of74.37° in the third direction (x-direction) so that repetitive pitchesin the first and second directions are 207.68 μm and the repetitivepitch in the third direction is 225.42 μm and the crossing angle betweenthe first and second directions is 54.86° to form a mother die in whichmany convex triangular-pyramidal cube corners whose reflective elementshave a height of 100 μm are arranged on the brass plate in theclosest-packed state. The optical-axis tilt angle θ of the cube-cornerretroreflective elements is +4° and three prism faces constituting atriangular pyramid are all 90°.

[0115] A triangular-pyramidal cube-corner retroreflective sheeting madeof polycarbonate resin is formed in accordance with the same method asthe case of Embodiment 1.

[0116] Table 1 shows values of retroreflection coefficients of thetriangular-pyramidal retroreflective sheetings formed for saidEmbodiments 1 to 4 and Comparative Examples 1 and 2 (units ofreflectivities are all cd/L_(x)*m²).

[0117] A retroreflection coefficient is measured in accordance with thephotometric measuring method specified in JIS Z8714Retroreflectors-Optical Properties-Measuring Method to measurecombinations of observation angles and entrance angles as 0.2°/5° and0.2°/30°. Moreover, for a rotation angle of a measurement sample, thedirection of a third-directional V-groove is decided as 0° and thedirection obtained by rotating the measurement sample by 90° from thedirection of the V-groove is decided as a rotation angle of 90° toperform measurement.

[0118] To observe entrance angularities of the triangular-pyramidalretroreflective sheetings prepared for said Embodiments 1 to 4 andComparative Examples 1 and 2, retroreflection coefficients of samplesare measured by setting an observation angle to a constant value of 0.2°and changing entrance angles from 5° to 10°, 15°, 20°, 25°, and 30° andshown in FIG. 6 by assigning entrance angles to the abscissa and valuesobtained by dividing retroreflection coefficients at various entranceangles by the retroreflection coefficient at an entrance angle of 5° tothe ordinate as brightness change rates.

[0119] Moreover, to observe rotation angularities of the above samples,retroreflection coefficients of various samples are measured by settingan observation angle to a constant value of 0.2° and an entrance angleto a constant value of 5° and changing rotation angles from 0° to 180°and shown in FIG. 7 by assigning rotation angles to the abscissa andvalues obtained by retroreflection coefficients at various rotationangles by the maximum retroreflection coefficient of each sample to theordinate as brightness change rates.

[0120] As shown in the following Table 1 and FIGS. 6 and 7, the decreaseof retroreflection coefficients is small even at a high entrance angleand moreover, the change of retroreflection coefficients is small for achange of rotation angles in the case of the retroreflective sheetingsof Embodiments 1 to 4 of the present invention while the brightness isextremely deteriorated at an entrance angle of 15° in the case of theretroreflective sheeting prepared for Comparative Example 1 and thebrightness is extremely deteriorated at a rotation angle of 90° in thecase of the reflective sheeting prepared for the Comparative Example 2.TABLE 1 Measurement Measurement Measurement condition 1 condition 2condition 3 Rotation angle 0° 0° 90° Observation angle 0.2° 0.2 0.2°Entrance angle 5° 30° 5° Embodiment 1 1434 645 1045 Embodiment 2 1365734 1085 Embodiment 3 1230 612 1118 Embodiment 4 1204 682 1124Comparative 1683 392 1156 Example 1 Comparative 1139 724 348 Example 2

[0121] The present invention is a triangular-pyramidal cube-cornerretroreflective element characterized in that triangular-pyramidalcube-corner retroreflective elements protruding on a common bottom plane(S_(x)-S_(x)′) share a base edge (x) on the bottom plane (S_(x)-S_(x)′)and are arranged on the bottom plane (S_(x)-S_(x)′) so as to be facedeach other, the bottom plane (S_(x)-S_(x)′) is a common plane includingmay base edges (x,x, . . . ) shared by the triangular-pyramidalreflective elements, the two triangular-pyramidal reflective elementsfaced each other include the shared base edges (x,x, . . . ) on thebottom plane (S_(x)-S_(x)′) and form a pair of substantially-same-shapeelements faced each other so as to be substantially symmetric to planes(L_(x)-L_(x), L_(x)-L_(x), . . .) vertical to the bottom plane(S_(x)-S_(x)′), and the optical axis of the triangular-pyramidalreflective elements tilts so that the angle formed between the opticalaxis and the vertical line becomes 0.5 to 1.5° in the direction in whichthe difference (q−p) between the distance (q) from the intersection (Q)of the optical axis and the bottom plate (S_(x)-S_(x)′) up to the baseedges (x,x, . . . ) shared by the element pair and the distance (p) fromthe intersection (P) of a vertical line extended from the apex of theelement pair to the bottom plane (S_(x)-S_(x)′) up to the base edges(x,x, . . . ) shared by the element pair becomes plus (+) or minus (−).

[0122] Thereby, a retroreflective sheeting of the present invention isnot only superior in high-brightness characteristic which is a basicoptical characteristic generally requested for a triangular-pyramidalreflective element, that is, reflectivity represented by thereflectivity of the light incoming from the front of thetriangular-pyramidal reflective element but also the entrance angularityand rotation angularity are greatly improved.

1. A triangular-pyramidal cube-corner retroreflective elementcharacterized in that triangular-pyramidal cube-corner retroreflectiveelements protruded onto a common bottom plane (S_(x)-S_(x)′) share abase edge (x) on the bottom plane and are arranged on the bottom plane(S_(x)-S_(x)′) in the closest-packed state so as to be faced each other,the bottom plane (S_(x)-S_(x)′) is a common plane including many baseedges (x,x, . . . ) shared by the triangular-pyramidal reflectiveelements, two faced triangular-pyramidal reflective elements include theshared base edges (x,x, . . . ) on the bottom plane (S_(x)-S_(x)′) andform a pair of substantially-same-shaped elements faced each other so asto be substantially symmetric to planes (L_(x)-L_(x), L_(x)-L_(x), . . .) vertical to the bottom plane (S_(x)-S_(x)′), and the optical axis ofthe triangular-pyramidal reflective element pair tilts so that the angleformed between the optical axis and a vertical line extended from apexes(H₁ and H₂) of the elements to the bottom plane (S_(x)-S_(x)′) rangesbetween 0.5° and 1.5° in the direction in which the difference (q−p)between the distance (q) from the intersection (Q) of the optical axisand the bottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . )shared by the elements and the distance (p) from the intersection (P) ofthe vertical line and the bottom plane (S_(x)-S_(x)′) up to the baseedges (x,x, . . . . ) shared by the element pair becomes plus (+) orminus (−).
 2. The triangular-pyramidal cube-corner retroreflectiveelement according to claim 1, characterized in that the optical axis viaapexes (H₁ and H₂) of the triangular-pyramidal reflective elements tiltsby 0.6° to 1.4° from a vertical line extended from the apexes (H₁ andH₂) of the above triangular-pyramidal reflective elements to the bottomplane (S_(x)-S_(x)′) in the direction for (q−p) to become plus (+) orminus (−).
 3. The triangular-pyramidal cube-corner retroreflectiveelement according to any one of claims 1 and 2, characterized in thatthe optical axis of the triangular-pyramidal reflective elements tiltsby 0.6° to 1.4° in the direction in which the difference (q−p) betweenthe distance (q) from the intersection (Q) of the optical axis and thebottom plane (S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared bythe elements and the distance (p) from the intersection (P) of avertical line extended from apexes (H₁ and H₂) of the elements to thebottom plane (S_(x)-S_(x)′) and the bottom plane (S_(x)-S_(x)′) up tothe base edges (x,x, . . . . ) shared by the elements becomes plus (+).4. The triangular-pyramidal cube-corner retroreflective elementaccording to any one of claims 1 to 3, characterized in that h_(x) issubstantially larger than h_(y) and h_(z) when assuming the height froma bottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) sharedby two triangular-pyramidal reflective elements faced each other up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements ash_(x), the height from a bottom plane (S_(y)-S_(y)′) including the otherbase edges (y,y, . . . ) of the triangular-pyramidal reflective elementsup to apexes (H₁ and H₂) of the triangular-pyramidal reflective elementsas h_(y), and the height from a bottom plane (S_(z)-S_(z)′) includingthe still other base edges (z,z, . . . ) of the triangular-pyramidalreflective elements up to apexes (H₁ and H₂) of the triangular-pyramidalreflective elements as h_(z).
 5. The triangular-pyramidal cube-cornerretroreflective element according to any one of claims 1 to 4,characterized in that h_(y) and h_(z) are substantially equal to eachother and h_(x) is substantially larger than h_(y) and h_(z) whenassuming the height from a bottom plane (S_(x)-S_(x)′) including baseedges (x,x, . . . ) shared by two triangular-pyramidal reflectiveelements faced each other up to apexes (H₁ and H₂) of thetriangular-pyramidal reflective elements as h_(x), the height from abottom plane (S_(y)-S_(y)′) including the other base edges (y,y, . . . )of the triangular-pyramidal reflective elements up to apexes (H₁ and H₂)of the triangular-pyramidal reflective elements as h_(y), and the heightfrom a bottom plane (S_(z)-S_(z)′) including the still other base edges(z,z, . . . ) of the triangular-pyramidal reflective elements up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements ashz.
 6. The triangular-pyramidal cube-corner retroreflective elementaccording to either of claims 1 and 2, characterized in that the opticalaxis of the triangular-pyramidal reflective elements tilts in thedirection in which the difference (q−p) between the distance (q) fromthe intersection (Q) of the optical axis and the bottom plane(S_(x)-S_(x)′) up to the base edges (x,x, . . . ) shared by the elementpair and the distance (p) from the intersection (P) of a vertical lineextended from apexes (H₁ and H₂) of the elements to the bottom plane(S_(x)-S_(x)′) and the bottom plane (S_(x)-S_(x)′) up to the base edges(x,x, . . . ) shared by the elements becomes minus (−) and moreover,h_(y) and h_(z) are substantially equal to each other and h_(x) issubstantially smaller than h_(y) and h_(z) when assuming the height froma bottom plane (S_(x)-S_(x)′) including base edges (x,x, . . . ) sharedby two triangular-pyramidal reflective elements faced each other up toapexes (H₁ and H₂) of the triangular-pyramidal reflective elements ash_(x), the height from a bottom plane (S_(y)-S_(y)′) including the otherbase edges (y,y, . . . ) of the triangular-pyramidal reflective elementsup to apexes (H₁ and H₂) of the triangular-pyramidal reflective elementsas h_(y), and the height from a bottom plane (S_(z)-S_(z)′) includingthe still other base edges (z,z, . . . ) of the triangular-pyramidalreflective elements up to apexes (H₁ and H₂) of the triangular-pyramidalreflective elements as h_(z).
 7. The triangular-pyramidal cube-cornerretroreflective element according to any one of claims 1 to 6,characterized in that an inequality 1.03<h_(max)/h_(min)<1.3 issatisfied when at least two of the above h_(x), h_(y), and h_(z) aresubstantially different from each other and the maximum one of theh_(x), h_(y), and h_(z) is assumed as h_(max) and the minimum one ofthem is assumes as h_(min).
 8. The triangular-pyramidal cube-cornerretroreflective element according to any one of claims 1 to 7,characterized in that the above h_(x), h_(y), and h_(z) respectivelyrange between 50 and 500 μm.
 9. The triangular-pyramidal cube-cornerretroreflective element according to any one of claims 1 to 7,characterized in that the above hx, hy, and hz respectively rangebetween 60 and 200 μm.
 10. The triangular-pyramidal cube-cornerretroreflective element according to any one of claims 1 to 9,characterized in that at least one prism-face angle formed when threelateral faces (faces a₁, b₁, and c₁) or (faces a₂, b₂, and C₂) of thetriangular-pyramidal cube-corner retroreflective element cross eachother ranges between 89.5° and 90.5° and is slightly deviated from90.000°.
 11. The triangular-pyramidal cube-corner retroreflectiveelement according to any one of claims 1 to 10, characterized in thatthe above two triangular-pyramidal cube-corner retroreflective elementsfaced each other are arranged in the closest-packed state and formedlike a sheet while sharing base edges (x,x, . . . ).